1. Ph.D. Candidate, College of Management and Economics, Tianjin Univ., No. 92 Nankai District, Tianjin 300072, P.R. China.
2. Lecturer, School of Economics and Management, Beihang Univ., Haidian District, Beijing 100191, P.R. China (corresponding author).
3. Professor, College of Management and Economics, Tianjin Univ., No. 92 Nankai District, Tianjin 300072, P.R. China.